Direct measurement of shear-induced cross-correlations of Brownian motion

TL;DR

This paper experimentally and theoretically investigates shear-induced cross-correlations in Brownian motion of particles, revealing time-asymmetry and anti-correlations caused by shear flow and hydrodynamic interactions.

Contribution

It provides the first direct measurements of shear-induced cross-correlations and confirms their behavior with a Langevin model, including novel anti-correlation effects.

Findings

Strong time-asymmetry in cross-correlations due to shear flow

Detection of shear-induced anti-correlation between particles

Experimental validation of Langevin model predictions

Abstract

Shear-induced cross-correlations of particle fluctuations perpendicular and along stream-lines are investigated experimentally and theoretically. Direct measurements of the Brownian motion of micron-sized beads, held by optical tweezers in a shear-flow cell, show a strong time-asymmetry in the cross-correlation, which is caused by the non-normal amplification of fluctuations. Complementary measurements on the single particle probability distribution substantiate this behavior and both results are consistent with a Langevin model. In addition, a shear-induced anti-correlation between orthogonal random-displacements of two trapped and hydrodynamically interacting particles is detected, having one or two extrema in time, depending on the positions of the particles.